A wireless terminal measures and reports the signal strength of its serving cell and some number of neighboring cells as part of the handoff process. The frequency of these reports, the number of neighboring cells monitored by the wireless terminal, and the reporting criteria depend on the air interface protocol of the cellular network (e.g., IS-136, GSM, IS-95 CDMA, etc.). Since each cell in the network transmits a constant control signal, the strength of this signal at the wireless terminal is an indication of the distance from the cell's antenna to the wireless terminal. Thus, it is possible to derive an estimate of the location of the wireless terminal from the strength of the signals that it reports by comparing the reported signal strengths to a model of the signal environment.
The accuracy of the location estimates that can be obtained from reported signal strength measurements depends on many factors that can vary from location to location and include, for example:    the number of signal strength measurements reported;    the accuracy with which the wireless terminal can measure signal strength;    the accuracy with which the wireless terminal can report signal strength values to the switching center (i.e., quantization);    the accuracy of the signal strength model of the environment; and    local attenuation caused by obstructions (e.g., terrain, vehicles, trees, etc.).
In addition, the accuracy of location estimates based on signal strength measurements also depends on the sensitivity of the signal environment to changes in location. For example, if there is a region in which received signal strength is relatively insensitive to changes in location, then reported signal strength measurements at a wireless terminal in that region could result in a relatively inaccurate location estimate, even if the model of the signal environment were perfect. Consequently, estimates based on signal strength measurements alone might not be sufficiently accurate for a specific location-based application at all locations within a service area.